Real-time PCR amplification for enumeration of methanogenic archaeal 16S rRNA gene was performed using total bacterial 16S rRNA gene as reference gene according to Guo et al. (2008). Amplifications were performed in triplicate using a MJ Research PTC- 200™ Chromo4 thermocycler (MJ Research) in a 25-ml mixture containing 2 SsoAdvanced Universal SYBR®Green Supermix (Bio- Rad), 400 nmol L 1 of each primer (Table 2) and 5 ng of template DNA. The amplification conditions involved denaturation at 95 C for 3 min, followed by 40 cycles of 95 C for 15 s and 60 C for for 30 s and fluorescence data were collected at the end of the hybridization step. Optimization of assay conditions were performed for both primers and template DNA concentration and a linear regression of the threshold cycle (CT) for different DNA dilution vs the log dilution of pooled DNA was used to estimate PCR efficiency (E ¼ 10 1/slope) for each primer pair (Pfaffl, 2001) where “5” is the fold dilution. Negative DNA standards were included in each qPCR run to prove specificity of the primer pairs. Amplicon specificity was tested with a dissociation curve analysis by increasing the temperature of 0.5 C every 30 s from 65 to 95 C. Data output released by Opticon Monitor software (version 2.03 MJ Research). The relative abundance of methanogenic archaeal community was calculated using the 2 DDCT method (Pfaffl, 2001) because efficiencies of the amplification curves of the bacterial and archaeal 16S genes were equal (E ¼ 1.96). For each sample, a DCT value was calculated subtracting the bacterial 16S CT (reference gene) from the archaeal 16S CT (target gene). DCT values from PF-2 soil samples were averaged and used as calibrator (Pfaffl, 2001) in the equation: DDCT ¼ DCT(given sample) DCT(calibrator). Soils with a 2 DDCT value below 1 have lower than PF-2 methanogenic archaeal abundance, while soils with a 2 DDCT value above 1 have higher than PF-2 methanogenic archaeal abundance.